Biofilms are surface attached microbial communities that predominates infection sites, and are of keen medical importance as they resist killing by antibiotics and the immune system. Progress in understanding how biofilm cells interact to elicit phenotypic changes has been impeded by limited strategies to probe their cellular interactions. Here, we seek to fill-in knowledge gaps by describing a novel process where cells within biofilms exchange their outer membrane (OM) lipoproteins, which result in phenotypic changes. Our model organism, Myxococcus xanthus, is a social gram-negative bacterium that can undergo multicellular development, is used to probe biofilm cellular dynamics. In preliminary results we now show that OMs are also exchanged and we identify cellular proteins required for transfer. We also show that transfer, which only occurs in structured biofilms, involves kin recognition whereby cells distinguish themselves from other closely related M. xanthus isolates. As transfer involves costly bulk movement of OM material, we believe this exchange process is a form of cooperative behavior where cells communicate and share resources. Lastly, we show transfer regulates swarm expansion and may be mediated by nanotube structures. Our results have broad implications as eukaryotic cells are widely known to exchange cellular components, and in bacterial systems, protein exchange is beginning to be appreciated as a prevalent process involved in communication and diverse cellular processes. Our genetic system is well poised to tackle these fundamental issues. Here, in Aim 1 we will use genetic approaches to identify the cellular complement of proteins involved in transfer, and define their interactions an the role nanotubes might play in transfer. Aim 2 will characterize the mechanism of kin recognition in transfer and define the genetic determinant for cell-cell recognition. Aim 3 will determine how OM exchange regulates swam expansion and a role it might play in development and envelope homeostasis. These combined efforts will advance our mechanistic understanding of how biofilm cells interact, recognize one another and exchange cellular material, which results in phenotypic changes that are distinct from planktonic or isolated cells. PUBLIC HEALTH RELEVANCE: Biofilms are dense microbial mats that are medically important as they account for most microbial infections. Biofilm infections are difficult and expensive to treat because they are resistant to the host immune system and antimicrobial agents. This proposal seeks to investigate novel cell interactions in biofilms, which may advance our understanding of biofilms and consequently lead to improved intervention strategies.